Trucks are often used to transport large loads of cargo. In some situations the operator of a truck can maximize the amount of cargo hauled during a trip by carrying a load mounted on the truck and drawing a pull trailer carrying additional cargo. For example, a transfer dump truck can tow a pull trailer that has an additional dump truck body mounted to it. In this way the truck can carry nearly twice as much cargo to or from a work site in a single trip. Local and/or federal laws limit the weight of the load a truck is allowed to carry based on the number of axles and the spacing between the axles. The use of a pull trailer allows a truck to carry a larger load than the law would normally permit the truck to carry alone.
Dump trucks have often made use of such functionality by towing a pull trailer. An advantage of using a pull trailer is that a dump truck can transport a large load because of the extended wheel base of the truck and pull trailer, and yet can still have good maneuverability at the dump site because of the short wheel base of the dump truck. Two kinds of pull trailers are pup trailers and transfer trailers. A pup trailer typically has its own hydraulics to facilitate dumping of a load. In contrast, a transfer trailer does not have dumping hydraulics. The body of a transfer trailer is transferred into the dumping body on the truck in order for the load to be dumped.
A pull trailer is usually secured to a dump truck with a drawbar. The drawbar is typically fairly long, for example fifteen feet, in order to accomplish the necessary spacing between the dump truck and pull trailer. This spacing is required to provide the proper axle and wheel loads so that the loads of the two vehicles are not localized over too short a length, which would jeopardize bridges and other roadways susceptible to damage from excessive loading. In addition, most vehicle codes or similar statutes in various jurisdictions governing the load and axle limits of trucks and trailers require certain axle spacing and vehicle lengths.
Although a lengthy trailer drawbar is necessary to meet certain requirements under load conditions, such a drawbar adversely affects the maneuverability and safety of the combined dump truck and pull trailer. In particular, a lengthy drawbar presents an extremely dangerous situation for unwary motorists, cyclists, and pedestrians who fail to notice the drab, low-slung drawbar and become caught in the interval or gap between the dump truck and the pull trailer. For example, a driver interpreting visual cues in order to maneuver in traffic is likely to perceive the end of the dump truck as just that, the end of the dump truck. Changing lanes behind a truck-trailer combination with few or no visual cues other than a long, low drawbar that is not visible to the driver could cause the driver to become caught between the truck and pull trailer, or to collide with the drawbar. Likewise, a pedestrian preparing to cross the street may step off the curb after a dump truck has passed, unaware that a pull trailer is in tow, and be hit by the pull trailer. This type of accident occurred in Redmond, Wash., on Oct. 19, 2007, when a 21-year old man was hit and killed by the rear trailer of a dump truck. The majority of these collisions are attributable to the driver's (or pedestrian's) failure to notice the drawbar and/or the connected pull trailer. Accordingly, many accidents could be prevented by providing better visual and/or audible indicators of the trailer.